This invention relates to a process for preparing a brominated polystyrene which is useful as a flame-retardant for a synthetic resin.
A brominated polystyrene has been used as a flame-retardant for various synthetic resins represented by engineering plastics such as a saturated polyester and a polyamide. These resins into which a flame-retardant is formulated have been used in various fields such as electric and electronic fields where flame retardance is required.
Various processes for preparing a brominated polystyrene have been proposed. For example, there have been proposed a process for preparing a brominated polystyrene by reacting a polystyrene having a weight average molecular weight of 20,000 or more with bromine chloride in an organic solvent under anhydrous conditions in the presence of a metal salt of a Lewis acid as a brominating catalyst at a temperature of about 50.degree. C. or lower in Japanese Patent Publication No. 57684/1989 and a process for preparing a brominated polystyrene by brominating a polystyrene in the copresence of a dehydrating agent in aliphatic halogenated hydrocarbon as a solvent in the presence of a Lewis acid catalyst at 0.degree. to 40.degree. C. in Japanese Provisional Patent Publication No. 215807/1990. In either of these methods, a Lewis acid is used as a brominating catalyst so that it is required to carry out bromination under anhydrous conditions or at least under a condition that a water content in a reaction solvent is 0.02% or less. If the water content is more than 0.02%, the Lewis acid catalyst is decomposed so that bromination does not proceed as expected, whereby it is impossible to obtain a brominated polystyrene having a high bromine content and excellent heat resistance, which can be used as a flame-retardant. Further, the Lewis acid catalyst is decomposed even by water in the air so that sufficient control for storage is required. From the above points, conventional preparation processes were not satisfactory as an industrial preparation process.